A common type of candle that sees widespread use consists of a wick embedded in a block of paraffin wax, where the wax provides the fuel for the burning candle. Paraffin has many properties that make it suitable for use in a candle. Paraffin as used in candles is typically highly refined and, at room temperature, is in a crystalline state. Crystalline paraffin is naturally white, and white paraffin candles are commonplace. Colorants may be added to these paraffin-based candles, to thereby achieve essentially any desired coloration. Paraffin-based candles are also rigid, and can be formed into a free-standing pillar structure. Such pillar candles, whether white or colored, can be placed atop candlesticks and the like, and are a popular consumer item. Paraffin is an inexpensive raw material, which makes it an economically-attractive ingredient for a candle. Perhaps unappreciated by consumers is that paraffin is also a good material from which to prepare a candle because it meets the rather stringent burning requirements for a candle, as discussed in more detail herein.
Paraffin-based candles have a significant drawback, however. Paraffin-based candles are neither transparent nor translucent, and in fact are opaque. Candle manufacturers have recognized an unmet need for transparent candles, and particularly transparent candles which can adopt the pillar form, ie., are rigid and self-supporting. Thus, the literature describes numerous attempts to prepare a satisfactory transparent pillar candle.
One approach has been to combine a thermoplastic polyamide resin with a solvent, where the polyamide resin is formed from dimer acid and acts as a gellant. Examples of this approach are found in, for example, U.S. Pat. Nos. 3,615,289; 3,645,705; 3,819,342 and 4,449,987. Candles made by this approach, which may be referred to as polyamide gel candles, have significant shortcomings which have limited their commercial acceptance. For example, such compositions are often not very transparent or even translucent, and may require the addition of a "clarifying agent" to achieve even a semi-transparent state (see, e.g., U.S. Pat. No. 3,819,342). In addition, such compositions are often not as hard as desired, and may require additives that increase stiffness and hardness in order to achieve even a short pillar form (see, e.g., U.S. Pat. No. 3,645,705), or are simply recommended for use in containers (see, e.g., U.S. Pat. No. 3,819,342).
Another significant problem with candles prepared with polyamide gellants is that they typically exhibit syneresis, where this term refers to the formation of liquid on the surface of a gel or colloidal suspension. In other words, droplets of solvent or other candle additive often form on the surfaces of a polyarmide gel candle. Syneresis is a highly undesirable property in a candle because, among other reasons, 1) consumers don't want to touch a wet, oily candle; 2) the candle becomes more brittle as oil escapes; and 3) the droplets of liquid solvent/additives tend to burn quite quickly once the candle is lit, giving the candle a torch-like quality.
Syneresis is particularly pronounced when the candle incorporates fragrance: the fragrance is frequently observed to exude from the candle and exacerbate the flaring problem (as discussed in, e.g., U.S. Pat. No. 3,615,289). Thus, for example, U.S. Pat. No. 3,645,805 suggests using only a small amount of fragrance in a polyamide gel candle, while U.S. Pat. No. 3,615,289 recommends up to about 2 percent fragrance. When relatively high amounts of fragrance are incorporated into either a polyamide gel candle or the block copolymer gel candles discussed below, the fragrance is typically observed to separate from the candle matrix and pool on the top of the candle. This segregation of the fragrance leads to a flaring or flashing problem with gelled fragranced candles, so that these candles tend to bum in an "out of control" manner, which the prior art has yet to solve. Polyamide gel candles also tend to exhibit blooming, where this term identifies the formation of opaque regions on the candle's surface. Blooming causes a significant aesthetic defect (opaque regions) in a candle that is supposed to be transparent.
Basically, the phenomena of syneresis and blooming reflect the fact that the components of a polyamide gel candle are not sufficiently compatible with one another to maintain a homogeneous state. The prior art has both recognized this problem and attempted to solve it by various means. Two such approaches are the judicious choice of solvent (see, e.g., U.S. Pat. No. 3,819,342), and/or including additives in the candle composition, such as "anti-flaring" compounds (see, e.g., U.S. Pat. No. 3,615,289). These approaches have not been very successful in providing a candle which is desired by the public.
There are several other problems associated with the gel candles prepared with polyamide gellants. One such problem is the failure of the candle to have and/or retain a completely colorless clear appearance. More specifically, it is observed that these gel candles will typically develop an undesirable yellow hue over time and/or with burning. Another problem is that when a colorant has been added to a polyamide gel candle, the initial color of the candle can fade, possibly due to a reaction between the organic components of the candle and the colorant. Also, polyamide gel candles are often observed to form an irreversibly crosslinked structure, which is undesirable because once the molten composition is poured into the form of a candle, it cannot be remelted and repoured in instances where the original candle contained a structural defect.
In general, polyamide gel candles have serious shortcomings, and have not received wide commercial acceptance. An alternative approach to preparing a clear gel candle is to disperse block copolymers in a clear oil, where a preferred block copolymer is a rubber. Disclosures of this approach are found in, for example, U.S. Pat. No. 5,578,089 and PCT International Application No. PCT/US96/13993. According to the '089 patent, at least two components selected from the group consisting of diblock copolymers, triblock copolymers, radial copolymers, multiblock polymers and mixtures thereof may be used in combination with hydrocarbon oil to form a gel candle. PCT/US96/13993 is also directed to the use of various block copolymers, preferably thermoplastic rubbers, in combination with mineral oil to form a gel suitable for molding into a candle. Candles prepared by these technologies purportedly overcome many of the problems observed with polyamide gel candles.
However, the '089 patent mentions that while the candle may be free standing at room temperature, this patent also recommends that the candle is preferably supplied in a container. The '089 patent comments that a container is desirable because of the gel-like nature of the candle itself and its potential flowability when heated. The gel-like nature of the block copolymer candles is similar to the gel-like nature of the polyamide candles discussed above in that while a "free-standing" structure may be formed from each of these gels, such structures are "free-standing" only in the sense that Jell-O.TM., a well-known gelled food product, is free-standing. Thus, a portion of Jell-O.TM. may be placed on a plate and will maintain itself at some height above the plate, without being contained. In this sense, Jell-O.TM. and prior art gels are indeed free-standing. However, consumers desire candles with a free-standing pillar structure, and while such a structure might be made from a material with the consistency of Jell-O.TM., it would only precariously hold the pillar structure--one little tap and the pillar would flop over.
While the prior art has attempted to increase the rigidity of a gel candle (see, e.g., methyl esters as a stiffness additive as disclosed in U.S. Pat. No. 3,645,705), these attempts have either failed to provide the desired rigidity or have hurt other properties such as clarity, freedom from syneresis, and a self-perpetuating flame. For example, attempts to increase the rigidity of a gel candle may take an approach that causes a concomitant increase in the viscosity of the molten gel. A more viscous molten gel will tend to incorporate and retain more air bubbles. The presence of air bubbles in a cooled candle will cause the candle to have a translucent, rather than the desired transparent appearance.
Another reason why gel candles are often recommended for use in containers as opposed to being free-standing is due to the potential flowability of a gel candle when it is heated. Thus, when a prior art gel candle prepared from block copolymers is lit, the present inventors have observed that the melted gel runs over the sides of the candle and onto the supporting table, which is commercially undesirable. The present inventors speculate that this may be due to the thermal transfer within the candle material. Thus, the high temperature near the flame and wick of a gel candle is rapidly and efficiently transferred throughout the candle, so that the entire surface becomes heated above its melting point, producing too much melted candle. In any event, potential flowability issues associated with a gel candle can be avoided by placing the candle in a container, which is the approach often recommended in the prior art.
The use of a container is also desirable because of the problem of separating a gel candle from a mold. The thermoplastic polyamides that have been used in the prior art to form gel candles are often also useful as adhesives. And the block copolymers that have been used to form gel candles are rubbers which are very sticky when molten and/or when mixed with oils. Thus, the gel candles of the prior art are not easily removed from a mold (demolded), but instead may hold on tenaciously to the mold and be freed only with concomitant disfiguration of the surface of the candle. This disfiguration greatly reduces the candle's clarity which is a primary object of the use of a gel candle.
Therefore, and despite significant attention, there remains an unmet need in the art for a clear, free-standing, rigid and fragranced candle. The present invention provides a gel structure which is well suited for the preparation of a clear candle. In addition, the gel structure of the present invention may incorporate various active ingredients, such as fragrance, which can be emitted from the gel over a sustained period of time, thus providing a fragrant article such as a candle or air freshener. These and other related advantages of the present invention are disclosed below.